# set of countable ordinals

by zefram_c
Tags: countable, ordinals
P: 260
I can't get a good intuitive grasp on this set. Folland defines it as follows:

 Quote by Folland, Real Analysis There is an uncountable well ordered set S such that Ix={y in S: y
My questions / problems with it are somewhat as follows:

1) I don't see how the set can be uncountable if any initial segment is countable.

2) How do we know that the element x0 used in the proof exists?
 PF Patron Sci Advisor Emeritus P: 16,094 If you want an analogy, consider the first infinite ordinal, $\omega$. While it is an infinite ordinal, but every initial segment is finite... if you can understand this, you should be able to understand the first uncountable ordinal. x0 exists because ordinals are well ordered. Any collection of ordinals has a smallest element.
 HW Helper Sci Advisor P: 9,371 Did you understand Hurkyl's response to your question? I.e. either you meant to ask: why does it follow logically that the statements given are true? (which is an immediate consequence of their definitions and of logic) or you meant to ask: "I understand the proof, but how can this be possible?" Hurkyl answered the second version of your question. re-reading, it seems your question 1) was the psychological one, and your 2) was a (tauto)logical one. ah yes, you said you wanted an intuitive grasp of the set, so Hurkyl understood you correctly.
P: 260

## set of countable ordinals

Thanks Hurkyl - after reading your response, I went back and checked. Turns out my understanding of well-ordering was incorrect; I now get the logic. The analogy gave me food for thought, so I will think on it for now and see if I can persuade myself that it works.

 Related Discussions Calculus & Beyond Homework 2 General Math 3 Set Theory, Logic, Probability, Statistics 2 Set Theory, Logic, Probability, Statistics 7 Set Theory, Logic, Probability, Statistics 2